Light valves, fluid suspension devices which control the transmission of light, have been known for many years. These devices usually operate in a manner so as to increase the transmission of radiation when the device is activated (by applying a field across the suspension), and return to a state of less transmission when the device is inactivated. There are many uses for such a light valve. However, there has always been a desire to have a light valve which operates so that when activated it would decrease the transmission of light from that in the inactive condition. In other words, as the cell was activated, the amount of light that could pass through would be less than before the cell was activated. Such a cell would be advantageous to have since it would provide a fail-safe feature in many applications. For example, for use in automobile windshields, automobile rear view mirrors, windows in buildings, space helmets, etc. It would be particularly useful to have a light valve that was clear or permitted substantial transmission of light in the inactive condition and was dark and prevented the transmission of light in the activated condition. Thus, in the aforementioned examples, if the light valve failed for some reason it would fail-safe since the light valve would become transparent in the inactive condition. Thus, the automobile windshield would be clear as well as the rear view mirror and the windows in the space helmets. If prior art fluid suspension light valves were used in the aforementioned examples, they would tend to fail in an unsafe condition. For example, if such a prior art light valve was used as a windshield in an automobile for the purpose of controlling glare and sunlight, and the electrical system which controls the windshield accidently failed while the vehicle was in motion, the light valve would become deactivated and dark and the driver's view would suddenly become blocked. On the other hand, if the light valve operated in the reverse mode, that is, normally clear in the inactive condition and dark in the activated condition, if the electrical system should fail the light valve would become deactivated and transmissive and thus the driver would have a clear view of the road. Likewise, a rear view mirror made in such a manner for use to control glare from headlights would also be fail-safe. The mirror would remain useful and clear even if the activating electrical system failed. In a similar manner, if such a reverse operating light valve were used as a window in a space helmet, the helmet would become clear if the electrical system failed. Similarly when such a reverse operating light valve is used for the windows in a building to control sunlight and glare the fail-safe feature operates so that if the light valve fails accidently light will still come in to illuminate the inside of the building. If the accidental deactivation of the valve were caused by the failure of the electrical power system in the buiding the fail-safe feature enables the interior to remain illuminated from the outside whereas otherwise the building would become dark.
There has been some attempt to make fluid suspension light valves which would become dark on the application of a voltage; however, these have operated in what can be described as a see-saw manner in that some parts of the spectrum become darker, while others simultaneously become more transmissive. A patent that discloses this is Rosenberg, U.S. Pat. No. 3,743,382. Briefly, when such a light valve is activated there is an increase in the transmission of radiation through the valve in one part of the electromagnetic spectrum and a decrease in transmisson in another part.
Thus, for the reasons aforementioned, it would be of substantial practical advantage to have light valves that operate in the reverse mode throughout the visible spectrum.